Immunotherapy has transformed cancer treatment, but therapy response ranges between 5% to 60% depending on tumor and treatment type. Accumulating evidence points to number and type of immune cell infiltrates within tumors as the culprit for failure. To enable higher success rates, novel treatments aim to prime tumors for immunotherapy by improving the immune milieu within the tumor microenvironment. However, there are currently no clinical technologies that can noninvasively assess immune contexture within tumors. Without noninvasive immunomonitoring methods, selection of patients, optimal dosing of immune-priming therapies, and optimal time to start immunotherapy will remain unsolved clinical challenges. To address the need for noninvasive immunomonitoring methods, we propose investigating utility of VisAcT, a T-cell specific PET agent, for longitudinal imaging of immune milieu in the tumor microenvironment. In this project, we will determine ability of VisAcT PET scans to provide information on the immune status of tumors prior to starting therapy and determine ability of VisAcT PET scans to evaluate changes in immune status of tumors during immune priming chemotherapy. If proven successful this imaging strategy has the potential to revolutionize almost every aspect of the immunotherapy field ? from drug discovery, through patient selection, to prevention of under- and overtreatment.